WO2017216445A1 - Method for making a gallium nitride light-emitting diode - Google Patents

Method for making a gallium nitride light-emitting diode Download PDF

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Publication number
WO2017216445A1
WO2017216445A1 PCT/FR2017/051400 FR2017051400W WO2017216445A1 WO 2017216445 A1 WO2017216445 A1 WO 2017216445A1 FR 2017051400 W FR2017051400 W FR 2017051400W WO 2017216445 A1 WO2017216445 A1 WO 2017216445A1
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layer
gallium nitride
nanowires
epitaxy
gan
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PCT/FR2017/051400
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French (fr)
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Ivan-Christophe Robin
Matthew Charles
Yohan Desieres
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Commissariat A L'energie Atomique Et Aux Energies Alternatives
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Priority to EP17734786.1A priority Critical patent/EP3472872A1/en
Priority to US16/306,971 priority patent/US10580931B2/en
Publication of WO2017216445A1 publication Critical patent/WO2017216445A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/005—Processes
    • H01L33/0062—Processes for devices with an active region comprising only III-V compounds
    • H01L33/0066—Processes for devices with an active region comprising only III-V compounds with a substrate not being a III-V compound
    • H01L33/007—Processes for devices with an active region comprising only III-V compounds with a substrate not being a III-V compound comprising nitride compounds
    • H—ELECTRICITY
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/005—Processes
    • H01L33/0093—Wafer bonding; Removal of the growth substrate
    • H—ELECTRICITY
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/16—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular crystal structure or orientation, e.g. polycrystalline, amorphous or porous
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/16—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular crystal structure or orientation, e.g. polycrystalline, amorphous or porous
    • H01L33/18—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular crystal structure or orientation, e.g. polycrystalline, amorphous or porous within the light emitting region
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/20—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate
    • H—ELECTRICITY
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/20—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate
    • H01L33/22—Roughened surfaces, e.g. at the interface between epitaxial layers
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/20—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate
    • H01L33/24—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate of the light emitting region, e.g. non-planar junction
    • H—ELECTRICITY
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/26—Materials of the light emitting region
    • H01L33/30—Materials of the light emitting region containing only elements of group III and group V of the periodic system
    • H01L33/32—Materials of the light emitting region containing only elements of group III and group V of the periodic system containing nitrogen
    • H—ELECTRICITY
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/44—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the coatings, e.g. passivation layer or anti-reflective coating
    • H01L33/46—Reflective coating, e.g. dielectric Bragg reflector
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    • H01L2933/00—Details relating to devices covered by the group H01L33/00 but not provided for in its subgroups
    • H01L2933/0008—Processes
    • H01L2933/0025—Processes relating to coatings
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    • H01L2933/0083—Periodic patterns for optical field-shaping in or on the semiconductor body or semiconductor body package, e.g. photonic bandgap structures
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/005—Processes
    • H01L33/0062—Processes for devices with an active region comprising only III-V compounds
    • H01L33/0075—Processes for devices with an active region comprising only III-V compounds comprising nitride compounds

Definitions

  • the present application relates to the field of optoelectronic devices. It relates more particularly to the production of light emitting diodes (LEDs) to gallium nitride (GaN).
  • LEDs light emitting diodes
  • GaN gallium nitride
  • a GaN LED consists essentially of a planar active stack of an N-type doped GaN layer or cathode layer, of an emissive layer with one or more quantum wells placed on and in contact with the layer.
  • a problem that arises is that some of the photons generated in the emitting layer are emitted in propagation directions forming with the normal at the stack an angle greater than the limit angle of total reflection on the upper and lower faces of the stack. These photons then remain confined within the LED, which limits the light output of the LED.
  • micro-structuring on the output face of the LED can be formed by chemical etching or by lithography and etching of the output face of the LED, after the formation of the active stack of the LED, or by texturing of the growth substrate prior to the formation of the LED. active stacking of the LED.
  • these methods have the disadvantage of degrading at least one layer of the LED, which can lead to degrade some of the characteristics of the LED, and / or to present limitations as to the form factors and / or the dimensions of the structures that can be performed.
  • an embodiment provides a method of manufacturing a gallium nitride light emitting diode, comprising the following successive steps:
  • a gallium nitride light emitting diode planar active stack having first and second doped gallium nitride layers of opposite conductivity types and, between the first and second gallium nitride layers, an emissive layer with one or more quantum wells;
  • the nanowires are made of gallium nitride.
  • the growth of the nanowires is carried out by vapor phase epitaxy in a silane-containing atmosphere.
  • the nanowires are made of zinc oxide.
  • the growth of the nanowires is carried out in a chemical bath.
  • the method further comprises forming a reflective structure on the side of the second layer of gallium nitride opposite the emissive layer.
  • the reflective structure is a Bragg mirror comprising only materials having a melting point greater than 1100 ° C.
  • the reflective structure is a metal layer.
  • step a) comprises a step of depositing the planar active stack by epitaxy on a growth substrate, and a step of transferring the planar active stack to a substrate for supporting and removing the growth substrate.
  • the thickness of the first layer of gallium nitride between the emitting layer and the base of the nanowires is less than the emission wavelength of the emitting layer.
  • the nanowires have a frustoconical shape of diameter gradually increasing away from the first layer of gallium nitride.
  • gallium nitride electroluminescent diode comprising:
  • planar active multilayer diode electro ⁇ luminescent gallium nitride having first and second gallium nitride doped layers of opposite conductivity type and, between the first and second gallium nitride layers, an emissive layer one or more wells quantum; and a plurality of nanowires disposed on the surface of the first layer of gallium nitride opposite the emissive layer.
  • the nanowires are made of gallium nitride or zinc oxide.
  • the nanowires have a frustoconical shape of diameter gradually increasing away from the first layer of gallium nitride.
  • the thickness of the first layer of gallium nitride between the emitting layer and the base of the nanowires is less than the emission wavelength of the emitting layer.
  • FIGS. 1A, 1B, 1C are sectional views illustrating steps of an example of an embodiment of a method for manufacturing a GaN LED
  • Figures 2A, 2B, 2C are sectional views illustrating steps of an alternative embodiment of the method of Figures 1A, 1B, 1C;
  • FIG. 3 is a sectional view illustrating another variant embodiment of a method for manufacturing an LED at
  • Figure 4 is a sectional view illustrating another alternative embodiment of a method of manufacturing a LEC GaN.
  • a method for producing a GaN LED comprising, after the formation of a planar active GaN LED stack, a step of growing nanowires on the face. output of the active stack of the LED.
  • the nanowire growth step does not cause any degradation of the previously formed layers of the active stack of the LED.
  • nanowires are particularly suitable structures for improving light extraction and / or controlling the emission directivity of a GaN LED.
  • nanowires are understood to mean yarns with a diameter of less than one micrometer, for example with a diameter of between 50 and 250 nm, and a length or height that may reach several micrometers, for example between 0.5 and 15 ⁇ m in length.
  • FIGS. 1A, 1B, 1C are sectional views illustrating steps of an example of an embodiment of a method for manufacturing a GaN LED.
  • FIG. 1A illustrates a formation step, on the upper face of a growth substrate 101, of a planar active stack 103 of GaN LEDs.
  • the substrate 101 is, for example, a substrate made of sapphire, corundum, silicon, or any other material on which a stack of gallium nitride-based layers can be deposited.
  • the GaN 103 active LED stack comprises, in order from the upper surface of the substrate, an N-type doped GaN layer or cathode layer 103a, an emitting layer 103b, and a layer doped GaN type P or anode layer 103c.
  • the emitting layer 103b is for example constituted by a stack of one or more emitting layers each forming a quantum well, for example based on GaN, InN, InGaN, AlGaN, AIN, AUnGaN , GaP, AlGaP or AlInGaP, and being each disposed between two barrier layers, for example based on GaN.
  • the lower face of the emitting layer 103b is in contact with the upper face of the cathode layer 103a
  • the upper face of the emitting layer 103b is in contact with the lower face of the anode layer 103c.
  • the stack 103 is adapted to emit photons from its emitting layer 103b when it is traversed by a current flowing from its anode layer 103c to its cathode layer 103a.
  • the active stack 103 is for example deposited by epitaxy on the growth substrate 101.
  • a buffer layer not shown, for example made of an AlN-GaN alloy, can interface between the growth substrate 101 and the lower GaN layer 103a. stacking.
  • FIG. 1B illustrates a step of transferring the active stack of LEDs to GaN 103 on a support substrate 105, for example a substrate made of sapphire, corundum, silicon, glass, etc., and then removing the substrate from During this step, the assembly comprising the growth substrate 101 and the active stack 103 can be turned over so as to orient the upper face (in the orientation of FIG. 1A) of the anode layer. 103c towards the upper face of the support substrate 105.
  • a reflective planar structure 107 is formed on the upper face of the support substrate 105 or on the upper face (in the orientation of FIG. 1A) of the layer 103c .
  • the reflecting structure 107 interfaces between the lower face of the layer 103c and the upper face of the substrate 105.
  • the growth substrate 101 is removed so as to discover the upper surface of the the layer of gallium nitride 103a.
  • the substrate 101 is for example removed by grinding and / or etching from its face opposite to the active stack 103.
  • the substrate 101 can be detached from the active stack 103 by means of a laser beam projected through the substrate 101 from its face opposite to the active stack 103 (by a laser lift-off method). More generally, any other method for removing the growth substrate 101 may be used.
  • an additional etching step may be provided to remove any buffer layers remaining on the upper side of the gallium nitride layer 103a.
  • part of the thickness of the gallium nitride layer 103a can be removed, for example by etching.
  • the output face of the GaN LED is its face opposite to the support substrate 105, corresponding to the upper face of the cathode layer 103a in the example shown.
  • the support substrate 105 may be transparent or opaque.
  • the function of the structure 107 is to reflect, towards the exit face of the LED, any photons emitted by the layer 103b towards the support substrate 105, in order to increase the luminous efficiency of the LED.
  • the reflective structure 107 is for example a Bragg mirror consisting of a stack of dielectric layers adapted to withstand high temperatures, for example greater than 1100 ° C. This advantageously allows the structure 107 to be able to support without degrading a subsequent step (FIG.
  • the reflecting structure comprises alternating layers of T1O2 (melting point of the order of 1800 ° C) and S1O2 (melting point of the order of 1700 ° C).
  • the upper layer of the structure 107, in contact with the lower face of the anode layer 103c, is preferably a conductive layer, for example a layer of ITO (indium tin oxide - melting point of the order 1500 to 1900 ° C), which advantageously facilitates the taking of an electrical contact on the anode layer 103c and improve the homogeneity of the electric current injected into the LED.
  • ITO indium tin oxide - melting point of the order 1500 to 1900 ° C
  • FIG. 1C illustrates a step subsequent to the step of FIG. 1B, during which nanowires 109 of GaN are grown on the upper face of the cathode layer 103a.
  • the nanowires 109 are formed by resumption of epitaxy on the upper face of the layer 103a.
  • the epitaxy conditions are selected capable of causing the growth of nanowires on the upper face of the layer 103a, in a direction substantially orthogonal to the upper face of the layer 103a.
  • the planar active stack previously produced is preferably such that, in the orientation of FIGS. 1B and 1C, the upper face of the GaN layer 103a (that is to say its face opposite the emitting layer 103b) is of nitrogen polarity.
  • the growth of GaN nanowires on a GaN substrate is indeed easier on the nitrogen polarity side of the substrate than on its gallium polarity side.
  • the growth of the nanowires 109 is, for example, made by MOVPE (metal-organic vapor phase epitaxy) in an atmosphere containing silane, for example at a temperature of about 1050.degree. ° C.
  • a mask having openings defining the growth zones of the nanowires 109 may optionally be formed on the upper face of the layer 103a prior to the step of growing the nanowires.
  • Anode and cathode contact metallizations may be formed in electrical contact respectively with the anode 103c and cathode 103a layers of the LED.
  • an anode contact can be taken from the upper face of the structure, in a peripheral zone of the LED not comprising the active layer 103b, the cathode layer 103a and the nanowires 109.
  • cathode can be taken from the upper face of the structure, in a peripheral zone of the LED does not include the nanowires 109.
  • an optoelectronic device comprising a plurality of identical or similar LEDs arranged on the same support substrate 105, for example to realize a display device of micro-screen type.
  • the dimensions and the positioning of the nanowires 109 may be adjusted according to the desired extraction characteristics and / or emission directivity.
  • this diameter can be calculated according to the teachings described in the book entitled "Optical Waveguide Theory" by AW Snyder and J. Love. This makes it possible to obtain a good emission directivity of the LED.
  • the wire diameter 109 may be of the order of 130 nm, with a distance between neighboring nanowires of at least 260 nm.
  • the thickness of the GaN layer 103a between the emitting layer 103b and the nanowires 109 is less than the emission wavelength of the emitting layer 103b.
  • the LED in the GaN (divided by the refractive index of GaN), for example at least ten times lower than the emission wavelength of the LED in the GaN.
  • FIGS. 2A, 2B, 2C are sectional views illustrating steps of an alternative embodiment of the method of FIGS. 1A, 1B, 1C, making it possible to facilitate the control of the thickness of the N-type doped GaN layer; extending between the emitting layer 103b and the nanowires 109.
  • This method comprises elements common with the method of FIGS. 1A, 1B, 1C. In the following, only the differences between the two processes will be highlighted.
  • FIG. 2A illustrates a step similar to the step described in relation to FIG. 1A, of formation, on the upper face of a growth substrate 101, of a stack 203 which differs from the stack 103 of FIG. 1A essentially in that, in the stack 203, the layer 103a of FIG. 1A is replaced by a stack 203a comprising, in order from the upper face of the substrate 101, a first N-type doped GaN layer 203a ] , an etch stop layer 203a2, and a second N-type doped GaN layer 203a3.
  • the active GaN LED stack is formed by the layers 203a3 (cathode layer), 103b (FIG. emitting layer) and 103c (anode layer).
  • the main function of the GaN layer 203a ] is to improve the mechanical strength and the quality of the epitaxy of the active stack.
  • the layer 203a2 is made of a material different from the GaN, for example aluminum nitride (AlN), and has the particular function of serving as an etch stop layer in a subsequent step (FIG. the layer of GaN 203a !.
  • FIG. 2B illustrates a step similar to the step described in relation with FIG. 1B, of transfer of the stack 203 on a support substrate 105, and then of withdrawal of the growth substrate 101.
  • the GaN 203a ] _ and the etch stop layer 203a2 are further removed so as to discover the upper face of GaN layer 203a3-
  • the removal of layer 203a ] _ is for example carried out by etching ICP-RIE (from English "Inductively Coupled Plasma Reactive Ion Etching" - inductively coupled ionic reactive plasma etching).
  • the removal of the layer 203a2 can also be carried out by chlorinated ICP-RIE etching, with detection of gallium by mass spectroscopy to stop the etching on the upper face of the layer 203a3.
  • FIG. 2C illustrates a step subsequent to the step of FIG. 2B, similar to the step described in relation with FIG. 1C, during which GaN nanowires 109 are grown on the upper face of the cathode layer.
  • Figure 3 illustrates an alternative embodiment of the method of Figures 1A, 1B, 1C.
  • This method comprises, for example, the same initial steps (FIGS. 1A and 1B) as in the example described with reference to FIGS. 1A, 1B, 1C, but differs from this example mainly in the shape of the GaN nanowires 109 made on the face the top of the cathode layer 103a of the LED.
  • the nanowires 109 have a frustoconical shape of diameter increasing progressively as one moves away from the upper face of the layer 103a. The progressive widening of the nanowires makes it possible to release the radial confinement of the electromagnetic field, thus limiting the diffraction at the end of the wire.
  • Such a frustoconical shape can for example be obtained by progressively reducing the epitaxial temperature as the nanowires grow, for example from a temperature of about 1050 ° C. at the beginning of epitaxy to a temperature of the order of 900 ° C at the end of epitaxy.
  • the distance between neighboring nanowires at the base of nanowires may be chosen greater or of the same order as the diameter of the nanowires at the end of the nanowires opposite to the layer 103a, and typically greater than 1 3 times the diameter of the wire at the level of the upper face of the layer 103.
  • the diameter of the nanowires 109 may be as follows: order of 130 nm at the base, and of the order of 1 ⁇ m at the opposite end to the layer 103a, with a distance between neighboring nanowires of the order of 1 ⁇ m.
  • the nanowires 109 may optionally coalesce at their opposite end to the layer 103a, so as to form a substantially flat continuous surface at their end opposite the layer 103a.
  • a transparent conductive layer for example made of ITO, or semitransparent, for example of metal, can be formed on and in contact with the upper surface of the nanowires, which advantageously makes it easier to take a contact electrical cathode and improve the homogeneity of the current injected into the LED.
  • Figure 4 illustrates another alternative embodiment of the method of Figures 1A, 1B, 1C.
  • This method comprises for example the same initial steps (FIGS. 1A and 1B) as in the example described with reference to FIGS. 1A, 1B, 1C, but differs from this example mainly in that, in the example of FIG. , the GaN nanowires 109 formed on the upper face of the cathode layer 103a are replaced by nanowires 409 zinc oxide (ZnO).
  • the growth of zinc oxide nanowires 409 is carried out in a low temperature chemical bath, for example at a temperature of between 60 and 150 ° C.
  • the growth of zinc oxide nanowires 409 is, for example, carried out by a process of the type described in the article entitled "Selective Area Growth of Well-Ordered ZnO Nanowire Arrays with Controllable Polarity" by Vincent Consonni et al. (ACS Nano, 2014, 8 (5), pp 4761-4770).
  • An advantage of the embodiment of Figure 4 is not to require a step of high temperature epitaxy to form the nanowires on the upper face of the LED.
  • the reflective structure 107 can then be formed by a single reflective metal layer, for example an indium-silver alloy. This makes it possible at the same time to obtain a good reflection coefficient of the photons, to take good quality electrical contact on the anode layer 103c, and to simplify the production of the structure 107 (with respect to the prediction of a mirror of Bragg).
  • FIGS. 3 and 4 can be adapted to the method of FIGS. 2A, 2B, 2C.
  • the conductivity types of the gallium nitride layers 103a, 203a3 (N type in the examples described) and 103c (P type in the examples described) can be reversed, the anode and cathode regions of the LEDs are then also reversed.
  • the described embodiments are not limited to the aforementioned examples in which the GaN LED active stack is made on a growth substrate 101, then transferred to a support substrate 105.
  • the substrate starting point may be a self-supporting GaN substrate, for example doped N-type, on one side of which the active stack 103 is epitaxial.
  • the active stack 103 is formed on the nitrogen polarity of the substrate .
  • GaN nanowires can be formed directly on the face of the active stack opposite to the substrate, which is a face of nitrogen polarity.
  • a reflective metal for example silver
  • this metal may be deposited before the growth of the nanowires over the entire surface of the layer 103a, and then removed locally in the growth zones of the nanowires.
  • the reflective metal may be deposited on the entire surface of the LED after the production of the nanowires 109, 409, for example by a conformal deposition method, then a directional etching step may be implemented to remove the metal on the upper surface of nanowires 109, 409.
  • the nanowires 109 are GaN
  • the layer 103b may be adapted to emit blue light
  • quantum wells adapted to convert yellow light part of the blue light emitted by the LED may be formed around the nanowires 109, so as to get an LED emitting white light.

Abstract

The invention relates to a method for making a gallium nitride light-emitting diode, comprising the following successive steps: a) forming a planar active stack (103) of gallium nitride light-emitting diodes comprising first (103a) and second (103c) layers of gallium nitride doped with opposite conductivity types and, between the first (103a) and second (103c) layers of gallium nitride, an emitting layer (103b) with one or more quantum wells; and b) growing nanowires (109) on the surface of the first layer (103a) of gallium nitride opposite the emitting layer (103b).

Description

PROCEDE DE FABRICATION D ' UNE DIODE ELECTROLUMINESCENTE AU  METHOD FOR MANUFACTURING A LIGHT-EMITTING DIODE
NITRURE DE GALLIUM  GALLIUM NITRIDE
La présente demande de brevet revendique la priorité de la demande de brevet français FR16/55678 qui sera considérée comme faisant partie intégrante de la présente description. The present patent application claims the priority of the French patent application FR16 / 55678 which will be considered as an integral part of the present description.
Domaine Field
La présente demande concerne le domaine des dispositifs optoélectroniques. Elle concerne plus particulièrement la réalisation de diodes électroluminescentes (LEDs) au nitrure de gallium (GaN) .  The present application relates to the field of optoelectronic devices. It relates more particularly to the production of light emitting diodes (LEDs) to gallium nitride (GaN).
Exposé de 1 ' art antérieur Presentation of the prior art
De façon classique, une LED au GaN est essentiellement constituée par un empilement actif planaire d'une couche de GaN dopé de type N ou couche de cathode, d'une couche émissive à un ou plusieurs puits quantiques disposée sur et en contact avec la couche de GaN dopée de type N, et d'une couche de GaN dopé de type P ou couche d'anode disposée sur et en contact avec la couche émissive. Le fonctionnement d'une telle LED repose sur l'émission de photons par recombinaison de paires électron-trou injectées électriquement dans la couche émissive.  In a conventional manner, a GaN LED consists essentially of a planar active stack of an N-type doped GaN layer or cathode layer, of an emissive layer with one or more quantum wells placed on and in contact with the layer. N-type doped GaN, and a P-type doped GaN layer or anode layer disposed on and in contact with the emissive layer. The operation of such an LED is based on the emission of photons by recombination electron-hole pairs electrically injected into the emitting layer.
Un problème qui se pose est que certains des photons générés dans la couche émissive sont émis selon des directions de propagation formant avec la normale à l'empilement un angle supérieur à l'angle limite de réflexion totale sur les faces supérieure et inférieure de l'empilement. Ces photons restent alors confinés à l'intérieur de la LED, ce qui limite le rendement lumineux de la LED. A problem that arises is that some of the photons generated in the emitting layer are emitted in propagation directions forming with the normal at the stack an angle greater than the limit angle of total reflection on the upper and lower faces of the stack. These photons then remain confined within the LED, which limits the light output of the LED.
Pour améliorer l'extraction lumineuse et/ou contrôler la directivité d'émission d'une LED au GaN, il a déjà été proposé de réaliser des micro-structurations sur la face de sortie de la LED. Ces micro-structurations peuvent être formées par attaque chimique ou par lithographie et gravure de la face de sortie de la LED, après la formation de l'empilement actif de la LED, ou encore par texturation du substrat de croissance préalablement à la formation de l'empilement actif de la LED. Ces méthodes ont toutefois pour inconvénients de dégrader au moins une couche de la LED, ce qui peut conduire à dégrader certaines des caractéristiques de la LED, et/ou de présenter des limitations quant aux facteurs de forme et/ou aux dimensions des structurations pouvant être réalisées.  To improve the light extraction and / or control the emission directivity of a GaN LED, it has already been proposed to carry out micro-structuring on the output face of the LED. These micro-structures can be formed by chemical etching or by lithography and etching of the output face of the LED, after the formation of the active stack of the LED, or by texturing of the growth substrate prior to the formation of the LED. active stacking of the LED. However, these methods have the disadvantage of degrading at least one layer of the LED, which can lead to degrade some of the characteristics of the LED, and / or to present limitations as to the form factors and / or the dimensions of the structures that can be performed.
Résumé summary
Ainsi, un mode de réalisation prévoit un procédé de fabrication d'une diode électroluminescente au nitrure de gallium, comprenant les étapes successives suivantes :  Thus, an embodiment provides a method of manufacturing a gallium nitride light emitting diode, comprising the following successive steps:
a) former un empilement actif planaire de diode électroluminescente au nitrure de gallium comportant des première et deuxième couches de nitrure de gallium dopées de types de conductivité opposés et, entre les première et deuxième couches de nitrure de gallium, une couche émissive à un ou plusieurs puits quantiques ; et  a) forming a gallium nitride light emitting diode planar active stack having first and second doped gallium nitride layers of opposite conductivity types and, between the first and second gallium nitride layers, an emissive layer with one or more quantum wells; and
b) faire croître des nanofils sur la surface de la première couche de nitrure de gallium opposée à la couche émissive.  b) growing nanowires on the surface of the first layer of gallium nitride opposite the emitting layer.
Selon un mode de réalisation, les nanofils sont en nitrure de gallium.  According to one embodiment, the nanowires are made of gallium nitride.
Selon un mode de réalisation, la croissance des nanofils est réalisée par épitaxie en phase vapeur dans une atmosphère contenant du silane. Selon un mode de réalisation, les nanofils sont en oxyde de zinc. According to one embodiment, the growth of the nanowires is carried out by vapor phase epitaxy in a silane-containing atmosphere. According to one embodiment, the nanowires are made of zinc oxide.
Selon un mode de réalisation, la croissance des nanofils est réalisée dans un bain chimique.  According to one embodiment, the growth of the nanowires is carried out in a chemical bath.
Selon un mode de réalisation, le procédé comprend en outre la formation d'une structure réfléchissante du côté de la face de la deuxième couche de nitrure de gallium opposée à la couche émissive.  According to one embodiment, the method further comprises forming a reflective structure on the side of the second layer of gallium nitride opposite the emissive layer.
Selon un mode de réalisation, la structure réflé- chissante est un miroir de Bragg comportant uniquement des matériaux ayant un point de fusion supérieur à 1100°C.  According to one embodiment, the reflective structure is a Bragg mirror comprising only materials having a melting point greater than 1100 ° C.
Selon un mode de réalisation, la structure réfléchissante est une couche métallique.  According to one embodiment, the reflective structure is a metal layer.
Selon un mode de réalisation, l'étape a) comprend une étape de dépôt de l'empilement actif planaire par épitaxie sur un substrat de croissance, et une étape de report de l'empilement actif planaire sur un substrat de support et de retrait du substrat de croissance.  According to one embodiment, step a) comprises a step of depositing the planar active stack by epitaxy on a growth substrate, and a step of transferring the planar active stack to a substrate for supporting and removing the growth substrate.
Selon un mode de réalisation, l'épaisseur de la première couche de nitrure de gallium entre la couche émissive et la base des nanofils est inférieure à la longueur d'onde d'émission de la couche émissive.  According to one embodiment, the thickness of the first layer of gallium nitride between the emitting layer and the base of the nanowires is less than the emission wavelength of the emitting layer.
Selon un mode de réalisation, les nanofils ont une forme tronconique de diamètre augmentant progressivement en s 'éloignant de la première couche de nitrure de gallium.  According to one embodiment, the nanowires have a frustoconical shape of diameter gradually increasing away from the first layer of gallium nitride.
Un autre mode de réalisation prévoit une diode électro¬ luminescente au nitrure de gallium, comprenant : Another embodiment provides a gallium nitride electroluminescent diode, comprising:
un empilement actif planaire de diode électro¬ luminescente au nitrure de gallium comportant des première et deuxième couches de nitrure de gallium dopées de types de conductivité opposés et, entre les première et deuxième couches de nitrure de gallium, une couche émissive à un ou plusieurs puits quantiques ; et une pluralité de nanofils disposés sur la surface de la première couche de nitrure de gallium opposée à la couche émissive . a planar active multilayer diode electro ¬ luminescent gallium nitride having first and second gallium nitride doped layers of opposite conductivity type and, between the first and second gallium nitride layers, an emissive layer one or more wells quantum; and a plurality of nanowires disposed on the surface of the first layer of gallium nitride opposite the emissive layer.
Selon un mode de réalisation, les nanofils sont en nitrure de gallium ou en oxyde de zinc.  According to one embodiment, the nanowires are made of gallium nitride or zinc oxide.
Selon un mode de réalisation, les nanofils ont une forme tronconique de diamètre augmentant progressivement en s 'éloignant de la première couche de nitrure de gallium.  According to one embodiment, the nanowires have a frustoconical shape of diameter gradually increasing away from the first layer of gallium nitride.
Selon un mode de réalisation, l'épaisseur de la première couche de nitrure de gallium entre la couche émissive et la base des nanofils est inférieure à la longueur d'onde d'émission de la couche émissive.  According to one embodiment, the thickness of the first layer of gallium nitride between the emitting layer and the base of the nanowires is less than the emission wavelength of the emitting layer.
Brève description des dessins Brief description of the drawings
Ces caractéristiques et leurs avantages, ainsi que d'autres, seront exposés en détail dans la description suivante de modes de réalisation particuliers faite à titre non limitatif en relation avec les figures jointes parmi lesquelles :  These and other features and advantages thereof will be set forth in detail in the following description of particular embodiments in a non-limiting manner with reference to the accompanying drawings in which:
les figures 1A, 1B, 1C sont des vues en coupe illustrant des étapes d'un exemple d'un mode de réalisation d'un procédé de fabrication d'une LED au GaN ;  FIGS. 1A, 1B, 1C are sectional views illustrating steps of an example of an embodiment of a method for manufacturing a GaN LED;
les figures 2A, 2B, 2C sont des vues en coupe illustrant des étapes d'une variante de réalisation du procédé des figures 1A, 1B, 1C ;  Figures 2A, 2B, 2C are sectional views illustrating steps of an alternative embodiment of the method of Figures 1A, 1B, 1C;
la figure 3 est une vue en coupe illustrant une autre variante de réalisation d'un procédé de fabrication d'une LED au FIG. 3 is a sectional view illustrating another variant embodiment of a method for manufacturing an LED at
GaN ; et GaN; and
la figure 4 est une vue en coupe illustrant une autre variante de réalisation d'un procédé de fabrication d'une LEC au GaN.  Figure 4 is a sectional view illustrating another alternative embodiment of a method of manufacturing a LEC GaN.
Description détaillée detailed description
De mêmes éléments ont été désignés par de mêmes références aux différentes figures et, de plus, les diverses figures ne sont pas tracées à l'échelle. Par souci de clarté, seuls les éléments qui sont utiles à la compréhension des modes de réalisation décrits ont été représentés et sont détaillés. En particulier, la réalisation des métallisations de contact sur les couches d'anode et de cathode des LEDs au GaN n'a pas été représentée, les modes de réalisation décrits étant compatibles avec les méthodes de réalisation et les dispositions usuelles des métallisations de contact d'anode et de cathode de LEDs au GaN. Dans la description qui suit, lorsque l'on fait référence à des qualificatifs de position absolue, tels que les termes "avant", "arrière", "haut", "bas", "gauche", "droite", etc., ou relative, tels que les termes "dessus", "dessous", "supérieur", "inférieur", etc., ou à des qualificatifs d'orientation, tels que les termes "horizontal", "vertical", etc., il est fait référence, sauf indication contraire, à l'orientation des vues en coupe correspondantes, étant entendu que, dans la pratique, les structures décrites peuvent être orientées différemment. Sauf précision contraire, les expressions "approximativement", "sensiblement", et "de l'ordre de" signifient à 10 % près, de préférence à 5 % près. The same elements have been designated with the same references in the various figures and, moreover, the various figures are not drawn to scale. For the sake of clarity, only the elements that are useful for understanding the described embodiments have been shown and are detailed. In In particular, the realization of the contact metallizations on the anode and cathode layers of the GaN LEDs has not been shown, the described embodiments being compatible with the usual methods and embodiments of the contact metallizations of FIG. anode and cathode of GaN LEDs. In the description which follows, when reference is made to absolute position qualifiers, such as the terms "before", "backward", "up", "down", "left", "right", etc., or relative, such as the terms "above", "below", "upper", "lower", etc., or with qualifiers for orientation, such as the terms "horizontal", "vertical", etc., it Reference is made, unless otherwise indicated, to the orientation of the corresponding sectional views, it being understood that, in practice, the described structures may be oriented differently. Unless otherwise specified, the terms "approximately", "substantially", and "of the order of" mean within 10%, preferably within 5%.
Selon un aspect d'un mode de réalisation, on prévoit un procédé de réalisation d'une LED au GaN, comportant, après la formation d'un empilement actif planaire de LED au GaN, une étape consistant à faire croître des nanofils sur la face de sortie de l'empilement actif de la LED. Un avantage est que l'étape de croissance de nanofils n' entraine aucune dégradation des couches préalablement formées de l'empilement actif de la LED. De plus, les nanofils constituent des structurations particulièrement adaptées à l'amélioration de l'extraction lumineuse et/ou au contrôle de la directivité d'émission d'une LED au GaN. On entend ici par nanofils des fils de diamètre inférieur au micromètre, par exemple de diamètre compris entre 50 et 250 nm, et de longueur ou hauteur pouvant atteindre plusieurs micromètres, par exemple de longueur comprise entre 0,5 et 15 um.  According to one aspect of an embodiment, there is provided a method for producing a GaN LED, comprising, after the formation of a planar active GaN LED stack, a step of growing nanowires on the face. output of the active stack of the LED. One advantage is that the nanowire growth step does not cause any degradation of the previously formed layers of the active stack of the LED. In addition, nanowires are particularly suitable structures for improving light extraction and / or controlling the emission directivity of a GaN LED. Here, nanowires are understood to mean yarns with a diameter of less than one micrometer, for example with a diameter of between 50 and 250 nm, and a length or height that may reach several micrometers, for example between 0.5 and 15 μm in length.
Les figures 1A, 1B, 1C sont des vues en coupe illustrant des étapes d'un exemple d'un mode de réalisation d'un procédé de fabrication d'une LED au GaN. La figure 1A illustre une étape de formation, sur la face supérieure d'un substrat de croissance 101, d'un empilement actif planaire 103 de LED au GaN. Le substrat 101 est par exemple un substrat en saphir, en corindon, en silicium, ou en tout autre matériau sur lequel un empilement de couches à base de nitrure de gallium peut être déposé. Dans cet exemple, l'empilement actif de LED au GaN 103 comprend, dans l'ordre à partir de la surface supérieure du substrat, une couche de GaN dopé de type N ou couche de cathode 103a, une couche émissive 103b, et une couche de GaN dopé de type P ou couche d'anode 103c. La couche émissive 103b est par exemple constituée par un empilement d'une ou plusieurs couches émissives formant chacune un puit quantique, par exemple à base de GaN, d'InN, d'InGaN, d'AlGaN, d'AIN, d'AUnGaN, de GaP, d'AlGaP ou d'AlInGaP, et étant chacune disposée entre deux couches barrière, par exemple à base de GaN. Dans cet exemple, la face inférieure de la couche émissive 103b est en contact avec la face supérieure de la couche de cathode 103a, et la face supérieure de la couche émissive 103b est en contact avec la face inférieure de la couche d'anode 103c. L'empilement 103 est adapté à émettre des photons à partir de sa couche émissive 103b lorsqu'il est traversé par un courant circulant de sa couche d'anode 103c vers sa couche de cathode 103a. L'empilement actif 103 est par exemple déposé par épitaxie sur le substrat de croissance 101. Une couche tampon non représentée, par exemple en un alliage AIN-GaN, peut faire interface entre le substrat de croissance 101 et la couche de GaN inférieure 103a de l'empilement. FIGS. 1A, 1B, 1C are sectional views illustrating steps of an example of an embodiment of a method for manufacturing a GaN LED. FIG. 1A illustrates a formation step, on the upper face of a growth substrate 101, of a planar active stack 103 of GaN LEDs. The substrate 101 is, for example, a substrate made of sapphire, corundum, silicon, or any other material on which a stack of gallium nitride-based layers can be deposited. In this example, the GaN 103 active LED stack comprises, in order from the upper surface of the substrate, an N-type doped GaN layer or cathode layer 103a, an emitting layer 103b, and a layer doped GaN type P or anode layer 103c. The emitting layer 103b is for example constituted by a stack of one or more emitting layers each forming a quantum well, for example based on GaN, InN, InGaN, AlGaN, AIN, AUnGaN , GaP, AlGaP or AlInGaP, and being each disposed between two barrier layers, for example based on GaN. In this example, the lower face of the emitting layer 103b is in contact with the upper face of the cathode layer 103a, and the upper face of the emitting layer 103b is in contact with the lower face of the anode layer 103c. The stack 103 is adapted to emit photons from its emitting layer 103b when it is traversed by a current flowing from its anode layer 103c to its cathode layer 103a. The active stack 103 is for example deposited by epitaxy on the growth substrate 101. A buffer layer, not shown, for example made of an AlN-GaN alloy, can interface between the growth substrate 101 and the lower GaN layer 103a. stacking.
La figure 1B illustre une étape de report de l'empilement actif de LED au GaN 103 sur un substrat de support 105, par exemple un substrat en saphir, en corindon, en silicium, en verre, etc., puis de retrait du substrat de croissance 101. Lors de cette étape, l'ensemble comportant le substrat de croissance 101 et l'empilement actif 103 peut être retourné, de façon à orienter la face supérieure (dans l'orientation de la figure 1A) de la couche d'anode 103c vers la face supérieure du substrat de support 105. Dans l'exemple représenté, préalablement au report de l'empilement actif 103 sur le substrat de support 105, une structure planaire réfléchissante 107 est formée sur la face supérieure du substrat de support 105 ou sur la face supérieure (dans l'orientation de la figure 1A) de la couche 103c. Ainsi, à l'issue du report, la structure réfléchissante 107 fait interface entre la face inférieure de la couche 103c et la face supérieure du substrat 105. Après le report, le substrat de croissance 101 est retiré de façon à découvrir la face supérieure de la couche de nitrure de gallium 103a. Le substrat 101 est par exemple retiré par meulage et/ou gravure à partir de sa face opposée à l'empilement actif 103. A titre de variante, dans le cas d'un substrat 101 transparent, par exemple un substrat en saphir ou en corindon, le substrat 101 peut être détaché de l'empilement actif 103 au moyen d'un faisceau laser projeté à travers le substrat 101 depuis sa face opposée à l'empilement actif 103 (par un procédé de type laser lift-off) . Plus généralement, toute autre méthode permettant de retirer le substrat de croissance 101 peut être utilisée. Après le retrait du substrat 101, une étape supplémentaire de gravure peut être prévue pour retirer d'éventuelles couches tampon subsistant du côté de la face supérieure de la couche de nitrure de gallium 103a. En outre, une partie de l'épaisseur de la couche de nitrure de gallium 103a peut être retirée, par exemple par gravure. FIG. 1B illustrates a step of transferring the active stack of LEDs to GaN 103 on a support substrate 105, for example a substrate made of sapphire, corundum, silicon, glass, etc., and then removing the substrate from During this step, the assembly comprising the growth substrate 101 and the active stack 103 can be turned over so as to orient the upper face (in the orientation of FIG. 1A) of the anode layer. 103c towards the upper face of the support substrate 105. In the example shown, previously at the postponement of the active stack 103 on the support substrate 105, a reflective planar structure 107 is formed on the upper face of the support substrate 105 or on the upper face (in the orientation of FIG. 1A) of the layer 103c . Thus, at the end of the transfer, the reflecting structure 107 interfaces between the lower face of the layer 103c and the upper face of the substrate 105. After the transfer, the growth substrate 101 is removed so as to discover the upper surface of the the layer of gallium nitride 103a. The substrate 101 is for example removed by grinding and / or etching from its face opposite to the active stack 103. Alternatively, in the case of a transparent substrate 101, for example a sapphire or corundum substrate the substrate 101 can be detached from the active stack 103 by means of a laser beam projected through the substrate 101 from its face opposite to the active stack 103 (by a laser lift-off method). More generally, any other method for removing the growth substrate 101 may be used. After removal of the substrate 101, an additional etching step may be provided to remove any buffer layers remaining on the upper side of the gallium nitride layer 103a. In addition, part of the thickness of the gallium nitride layer 103a can be removed, for example by etching.
Dans cet exemple, la face de sortie de la LED au GaN est sa face opposée au substrat de support 105, correspondant à la face supérieure de la couche de cathode 103a dans l'exemple représenté. Le substrat de support 105 peut être transparent ou opaque. La structure 107 a pour fonction de réfléchir, vers la face de sortie de la LED, d'éventuels photons émis par la couche 103b en direction du substrat de support 105, ceci afin d'augmenter le rendement lumineux de la LED. La structure réfléchissante 107 est par exemple un miroir de Bragg constitué d'un empilement de couches diélectriques adaptées à supporter des températures élevées, par exemple supérieures à 1100°C. Ceci permet avantageusement à la structure 107 de pouvoir supporter sans dégradation une étape ultérieure (figure 1C) de croissance de nanofils sur la face supérieure de la couche 103a, réalisée par épitaxie à haute température. A titre d'exemple, la structure réfléchissante comprend une alternance de couches de T1O2 (point de fusion de l'ordre de 1800 °C) et de S1O2 (point de fusion de l'ordre de 1700°C) . La couche supérieure de la structure 107, en contact avec la face inférieure de la couche d'anode 103c, est de préférence une couche conductrice, par exemple une couche d'ITO (oxyde d'indium étain - point de fusion de l'ordre de 1500 à 1900°C) , ce qui permet avantageusement de faciliter la prise d'un contact électrique sur la couche d'anode 103c et d'améliorer l'homogénéité du courant électrique injecté dans la LED. In this example, the output face of the GaN LED is its face opposite to the support substrate 105, corresponding to the upper face of the cathode layer 103a in the example shown. The support substrate 105 may be transparent or opaque. The function of the structure 107 is to reflect, towards the exit face of the LED, any photons emitted by the layer 103b towards the support substrate 105, in order to increase the luminous efficiency of the LED. The reflective structure 107 is for example a Bragg mirror consisting of a stack of dielectric layers adapted to withstand high temperatures, for example greater than 1100 ° C. This advantageously allows the structure 107 to be able to support without degrading a subsequent step (FIG. 1C) of growth of nanowires on the upper face of the layer 103a, carried out by epitaxial growth at high temperature. For example, the reflecting structure comprises alternating layers of T1O2 (melting point of the order of 1800 ° C) and S1O2 (melting point of the order of 1700 ° C). The upper layer of the structure 107, in contact with the lower face of the anode layer 103c, is preferably a conductive layer, for example a layer of ITO (indium tin oxide - melting point of the order 1500 to 1900 ° C), which advantageously facilitates the taking of an electrical contact on the anode layer 103c and improve the homogeneity of the electric current injected into the LED.
La figure 1C illustre une étape postérieure à l'étape de la figure 1B, au cours de laquelle on fait croître des nanofils 109 de GaN sur la face supérieure de la couche de cathode 103a. Les nanofils 109 sont formés par reprise d' épitaxie sur la face supérieure de la couche 103a. Les conditions d' épitaxie sont choisies aptes à provoquer la croissance de nanofils sur la face supérieure de la couche 103a, selon une direction sensiblement orthogonale à la face supérieure de la couche 103a. Pour cela, on utilise par exemple un procédé d' épitaxie du type décrit dans l'article intitulé "Homoepitaxial growth of catalyst-free GaN wires on N-polar substrates" de X. J. Chen et al. (applied physics letters 97, 151909 2010). L'empilement actif planaire préalablement réalisé est de préférence tel que, dans l'orientation des figures 1B et 1C, la face supérieure de la couche de GaN 103a (c'est-à-dire sa face opposée à la couche émissive 103b) soit de polarité azote. La croissance de nanofils de GaN sur un substrat de GaN étant en effet plus aisée sur la face de polarité azote du substrat que sur sa face de polarité gallium. La croissance des nanofils 109 est par exemple réalisée par MOVPE (de l'anglais "metal-organic vapor phase epitaxy" - épitaxie en phase vapeur aux organométalliques) dans une atmosphère contenant du silane, par exemple à une température de l'ordre de 1050°C. Pour contrôler le positionnement et les dimensions des nanofils 109, un masque comportant des ouvertures délimitant les zones de croissance des nanofils 109, par exemple un masque en nitrure de silicium, peut optionnellement être formé sur la face supérieure de la couche 103a préalablement à l'étape de croissance des nanofils. FIG. 1C illustrates a step subsequent to the step of FIG. 1B, during which nanowires 109 of GaN are grown on the upper face of the cathode layer 103a. The nanowires 109 are formed by resumption of epitaxy on the upper face of the layer 103a. The epitaxy conditions are selected capable of causing the growth of nanowires on the upper face of the layer 103a, in a direction substantially orthogonal to the upper face of the layer 103a. For this purpose, for example, an epitaxial method of the type described in the article entitled "Homoepitaxial Growth of Catalyst-free GaN Wires on N-polar substrates" by XJ Chen et al. (applied physics letters 97, 151909 2010). The planar active stack previously produced is preferably such that, in the orientation of FIGS. 1B and 1C, the upper face of the GaN layer 103a (that is to say its face opposite the emitting layer 103b) is of nitrogen polarity. The growth of GaN nanowires on a GaN substrate is indeed easier on the nitrogen polarity side of the substrate than on its gallium polarity side. The growth of the nanowires 109 is, for example, made by MOVPE (metal-organic vapor phase epitaxy) in an atmosphere containing silane, for example at a temperature of about 1050.degree. ° C. To control the positioning and dimensions of the nanowires 109, a mask having openings defining the growth zones of the nanowires 109, for example a silicon nitride mask, may optionally be formed on the upper face of the layer 103a prior to the step of growing the nanowires.
Des métallisations de contact d'anode et de de cathode, non représentées, peuvent être formées en contact électrique respectivement avec les couches d'anode 103c et de cathode 103a de la LED. A titre d'exemple, un contact d'anode peut être pris depuis la face supérieure de la structure, dans une zone périphérique de la LED ne comprenant pas la couche active 103b, la couche de cathode 103a et les nanofils 109. Un contact de cathode peut être pris depuis la face supérieure de la structure, dans une zone périphérique de la LED ne comprenant pas les nanofils 109. En pratique, on peut prévoir un dispositif optoélectronique comprenant une pluralité de LEDs identiques ou similaires disposées sur un même substrat de support 105, par exemple pour réaliser un dispositif d'affichage de type micro-écran.  Anode and cathode contact metallizations, not shown, may be formed in electrical contact respectively with the anode 103c and cathode 103a layers of the LED. By way of example, an anode contact can be taken from the upper face of the structure, in a peripheral zone of the LED not comprising the active layer 103b, the cathode layer 103a and the nanowires 109. cathode can be taken from the upper face of the structure, in a peripheral zone of the LED does not include the nanowires 109. In practice, there can be provided an optoelectronic device comprising a plurality of identical or similar LEDs arranged on the same support substrate 105, for example to realize a display device of micro-screen type.
Les dimensions et le positionnement des nanofils 109 peuvent être ajustés en fonction des caractéristiques d'extraction et/ou de directivité d'émission recherchées. A titre d'exemple, on peut prévoir des nanofils 109 présentant un diamètre sensiblement constant sur toute leur hauteur, ce diamètre étant choisi le plus élevé possible tout en restant suffisamment faible pour obtenir un guidage monomode de la lumière à la longueur d'onde d'émission de la couche émissive 103b. A titre d'exemple, ce diamètre peut être calculé selon les enseignements décrits dans l'ouvrage intitulé "Optical Waveguide Theory" de A.W. Snyder et J. Love. Ceci permet d'obtenir une bonne directivité d'émission de la LED. Par ailleurs, pour limiter les couplages parasites entre des nanofils voisins, on peut choisir de respecter une distance minimale entre les nanofils, par exemple une distance au moins égale à 1,3 fois le diamètre des nanofils. A titre d'exemple, pour une longueur d'onde d'émission de l'ordre de 400 nm, le diamètre des fils 109 peut être de l'ordre de 130 nm, avec une distance entre nanofils voisins d'au moins 260 nm. The dimensions and the positioning of the nanowires 109 may be adjusted according to the desired extraction characteristics and / or emission directivity. By way of example, it is possible to provide nanowires 109 having a substantially constant diameter over their entire height, this diameter being chosen as high as possible while remaining small enough to obtain a monomode guidance of the light at the wavelength of emitting the emitting layer 103b. By way of example, this diameter can be calculated according to the teachings described in the book entitled "Optical Waveguide Theory" by AW Snyder and J. Love. This makes it possible to obtain a good emission directivity of the LED. Furthermore, to limit parasitic coupling between neighboring nanowires, it is possible to choose to respect a minimum distance between the nanowires, for example a distance at least equal to 1.3 times the diameter of the nanowires. By way of example, for an emission wavelength of the order of 400 nm, the wire diameter 109 may be of the order of 130 nm, with a distance between neighboring nanowires of at least 260 nm.
De préférence, pour coupler efficacement la source lumineuse formée par la couche émissive 103b et les nanofils 109, l'épaisseur de la couche de GaN 103a entre la couche émissive 103b et les nanofils 109 est inférieure à la longueur d'onde d'émission de la LED dans le GaN (divisée par l'indice de réfraction du GaN), par exemple au moins dix fois plus faible que la longueur d'onde d'émission de la LED dans le GaN.  Preferably, in order to effectively couple the light source formed by the emitting layer 103b and the nanowires 109, the thickness of the GaN layer 103a between the emitting layer 103b and the nanowires 109 is less than the emission wavelength of the emitting layer 103b. the LED in the GaN (divided by the refractive index of GaN), for example at least ten times lower than the emission wavelength of the LED in the GaN.
Les figures 2A, 2B, 2C sont des vues en coupe illustrant des étapes d'une variante de réalisation du procédé des figures 1A, 1B, 1C, permettant de faciliter le contrôle de l'épaisseur de la couche de GaN dopé de type N s 'étendant entre la couche émissive 103b et les nanofils 109. Ce procédé comprend des éléments communs avec le procédé des figures 1A, 1B, 1C. Dans la suite, seules les différences entre les deux procédés seront mises en exergue.  FIGS. 2A, 2B, 2C are sectional views illustrating steps of an alternative embodiment of the method of FIGS. 1A, 1B, 1C, making it possible to facilitate the control of the thickness of the N-type doped GaN layer; extending between the emitting layer 103b and the nanowires 109. This method comprises elements common with the method of FIGS. 1A, 1B, 1C. In the following, only the differences between the two processes will be highlighted.
La figure 2A illustre une étape similaire à l'étape décrite en relation avec la figure 1A, de formation, sur la face supérieure d'un substrat de croissance 101, d'un empilement 203 qui diffère de l'empilement 103 de la figure 1A essentiellement en ce que, dans l'empilement 203, la couche 103a de la figure 1A est remplacée par un empilement 203a comprenant, dans l'ordre à partir de la face supérieure du substrat 101, une première couche de GaN dopé de type N 203a]_, une couche d'arrêt de gravure 203a2, et une deuxième couche de GaN dopé de type N 203a3- Dans cet exemple, l'empilement actif de LED au GaN est formé par les couches 203a3 (couche de cathode) , 103b (couche émissive) et 103c (couche d'anode). La couche de GaN 203a]_ a pour principale fonction d'améliorer la tenue mécanique et la qualité de l'épitaxie de l'empilement actif. La couche 203a2 est réalisée en un matériau différent du GaN, par exemple du nitrure d'aluminium (AIN), et a notamment pour fonction de servir de couche d'arrêt de gravure lors d'une étape ultérieure (figure 1B) de retrait de la couche de GaN 203a!. La figure 2B illustre une étape similaire à l'étape décrite en relation avec la figure 1B, de report de l'empilement 203 sur un substrat de support 105, puis de retrait du substrat de croissance 101. Lors de cette étape, la couche de GaN 203a]_ et la couche d'arrêt de gravure 203a2 sont en outre retirées de façon à découvrir la face supérieure de la couche de GaN 203a3- Le retrait de la couche 203a]_ est par exemple réalisé par gravure ICP-RIE (de l'anglais "Inductively Coupled Plasma Reactive Ion Etching" - gravure ionique réactive à plasma à couplage inductif) chlorée. Le retrait de la couche 203a2 peut lui aussi être réalisé par gravure ICP-RIE chlorée, avec détection de gallium par spectroscopie de masse pour arrêter la gravure sur la face supérieure de la couche 203a3-FIG. 2A illustrates a step similar to the step described in relation to FIG. 1A, of formation, on the upper face of a growth substrate 101, of a stack 203 which differs from the stack 103 of FIG. 1A essentially in that, in the stack 203, the layer 103a of FIG. 1A is replaced by a stack 203a comprising, in order from the upper face of the substrate 101, a first N-type doped GaN layer 203a ] , an etch stop layer 203a2, and a second N-type doped GaN layer 203a3. In this example, the active GaN LED stack is formed by the layers 203a3 (cathode layer), 103b (FIG. emitting layer) and 103c (anode layer). The main function of the GaN layer 203a ] is to improve the mechanical strength and the quality of the epitaxy of the active stack. The layer 203a2 is made of a material different from the GaN, for example aluminum nitride (AlN), and has the particular function of serving as an etch stop layer in a subsequent step (FIG. the layer of GaN 203a !. FIG. 2B illustrates a step similar to the step described in relation with FIG. 1B, of transfer of the stack 203 on a support substrate 105, and then of withdrawal of the growth substrate 101. During this step, the GaN 203a ] _ and the etch stop layer 203a2 are further removed so as to discover the upper face of GaN layer 203a3- The removal of layer 203a ] _ is for example carried out by etching ICP-RIE (from English "Inductively Coupled Plasma Reactive Ion Etching" - inductively coupled ionic reactive plasma etching). The removal of the layer 203a2 can also be carried out by chlorinated ICP-RIE etching, with detection of gallium by mass spectroscopy to stop the etching on the upper face of the layer 203a3.
La figure 2C illustre une étape postérieure à l'étape de la figure 2B, similaire à l'étape décrite en relation avec la figure 1C, au cours de laquelle on fait croître des nanofils de GaN 109 sur la face supérieure de la couche de cathode 203a3-FIG. 2C illustrates a step subsequent to the step of FIG. 2B, similar to the step described in relation with FIG. 1C, during which GaN nanowires 109 are grown on the upper face of the cathode layer. 203a3-
La figure 3 illustre une variante de réalisation du procédé des figures 1A, 1B, 1C. Ce procédé comprend par exemple les mêmes étapes initiales (figures 1A et 1B) que dans l'exemple décrit en relation avec les figures 1A, 1B, 1C, mais diffère de cet exemple principalement par la forme des nanofils en GaN 109 réalisés sur la face supérieure de la couche de cathode 103a de la LED. Dans l'exemple de la figure 3, les nanofils 109 ont une forme tronconique de diamètre croissant progressivement lorsque l'on s'éloigne de la face supérieure de la couche 103a. L'élargissement progressif des nanofils permet de relâcher le confinement radial du champ électromagnétique, limitant ainsi la diffraction en bout de fil. Ceci permet d'augmenter la directivité d'émission de la LED. Une telle forme tronconique peut par exemple être obtenue en réduisant progressivement la température d'épitaxie au fur et à mesure de la croissance des nanofils, par exemple depuis une température de l'ordre de 1050°C en début d'épitaxie jusqu'à une température de l'ordre de 900°C en fin d'épitaxie. La distance entre des nanofils voisins à la base des nanofils (c'est-à-dire au niveau de la face supérieure de la couche 103a) peut être choisie supérieure ou du même ordre que le diamètre des nanofils à l'extrémité des nanofils opposée à la couche 103a, et typiquement supérieure à 1,3 fois le diamètre du fil au niveau de la face supérieure de la couche 103. A titre d'exemple, pour une longueur d'onde d'émission de l'ordre de 400 nm, le diamètre des nanofils 109 peut être de l'ordre de 130 nm à la base, et de l'ordre de 1 ym à l'extrémité opposée à la couche 103a, avec une distance entre nanofils voisins de l'ordre de 1 ym. Les nanofils 109 peuvent éventuellement coalescer au niveau de leur extrémité opposée à la couche 103a, de façon à former une surface continue sensiblement plane au niveau de leur extrémité opposée à la couche 103a. Dans ce cas, une couche conductrice transparente, par exemple en ITO, ou semi- transparente, par exemple en métal, peut être formée sur et en contact avec la face supérieure des nanofils, ce qui permet avantageusement de faciliter la prise d'un contact électrique de cathode et d'améliorer l'homogénéité du courant injecté dans la LED. Figure 3 illustrates an alternative embodiment of the method of Figures 1A, 1B, 1C. This method comprises, for example, the same initial steps (FIGS. 1A and 1B) as in the example described with reference to FIGS. 1A, 1B, 1C, but differs from this example mainly in the shape of the GaN nanowires 109 made on the face the top of the cathode layer 103a of the LED. In the example of FIG. 3, the nanowires 109 have a frustoconical shape of diameter increasing progressively as one moves away from the upper face of the layer 103a. The progressive widening of the nanowires makes it possible to release the radial confinement of the electromagnetic field, thus limiting the diffraction at the end of the wire. This makes it possible to increase the emission directivity of the LED. Such a frustoconical shape can for example be obtained by progressively reducing the epitaxial temperature as the nanowires grow, for example from a temperature of about 1050 ° C. at the beginning of epitaxy to a temperature of the order of 900 ° C at the end of epitaxy. The distance between neighboring nanowires at the base of nanowires (that is to say at the upper face of the layer 103a) may be chosen greater or of the same order as the diameter of the nanowires at the end of the nanowires opposite to the layer 103a, and typically greater than 1 3 times the diameter of the wire at the level of the upper face of the layer 103. For example, for an emission wavelength of the order of 400 nm, the diameter of the nanowires 109 may be as follows: order of 130 nm at the base, and of the order of 1 μm at the opposite end to the layer 103a, with a distance between neighboring nanowires of the order of 1 μm. The nanowires 109 may optionally coalesce at their opposite end to the layer 103a, so as to form a substantially flat continuous surface at their end opposite the layer 103a. In this case, a transparent conductive layer, for example made of ITO, or semitransparent, for example of metal, can be formed on and in contact with the upper surface of the nanowires, which advantageously makes it easier to take a contact electrical cathode and improve the homogeneity of the current injected into the LED.
La figure 4 illustre une autre variante de réalisation du procédé des figures 1A, 1B, 1C. Ce procédé comprend par exemple les mêmes étapes initiales (figures 1A et 1B) que dans l'exemple décrit en relation avec les figures 1A, 1B, 1C, mais diffère de cet exemple principalement en ce que, dans l'exemple de la figure 4, les nanofils de GaN 109 formés sur la face supérieure de la couche de cathode 103a sont remplacés par des nanofils 409 en oxyde de zinc (ZnO) . A titre d' 'exemple, la croissance des nanofils 409 en oxyde de zinc est réalisée dans un bain chimique à basse température, par exemple à une température comprise entre 60 et 150 °C. La croissance des nanofils 409 en oxyde de zinc est par exemple réalisée par un procédé du type décrit dans l'article intitulé "Sélective Area Growth of Well-Ordered ZnO Nanowire Arrays with Controllable Polarity" de Vincent Consonni et al. (ACS Nano, 2014, 8 (5), pp 4761-4770). Un avantage de la variante de réalisation de la figure 4 est de ne pas nécessiter une étape d'épitaxie à haute température pour former les nanofils sur la face supérieure de la LED. La structure réfléchissante 107 peut alors être formée par une simple couche métallique réfléchissante, par exemple un alliage indium- argent. Ceci permet à la fois d'obtenir un bon coefficient de réflexion des photons, de prendre un contact électrique de bonne qualité sur la couche d'anode 103c, et de simplifier la réalisation de la structure 107 (par rapport à la prévision d'un miroir de Bragg) . Figure 4 illustrates another alternative embodiment of the method of Figures 1A, 1B, 1C. This method comprises for example the same initial steps (FIGS. 1A and 1B) as in the example described with reference to FIGS. 1A, 1B, 1C, but differs from this example mainly in that, in the example of FIG. , the GaN nanowires 109 formed on the upper face of the cathode layer 103a are replaced by nanowires 409 zinc oxide (ZnO). By way of example, the growth of zinc oxide nanowires 409 is carried out in a low temperature chemical bath, for example at a temperature of between 60 and 150 ° C. The growth of zinc oxide nanowires 409 is, for example, carried out by a process of the type described in the article entitled "Selective Area Growth of Well-Ordered ZnO Nanowire Arrays with Controllable Polarity" by Vincent Consonni et al. (ACS Nano, 2014, 8 (5), pp 4761-4770). An advantage of the embodiment of Figure 4 is not to require a step of high temperature epitaxy to form the nanowires on the upper face of the LED. The reflective structure 107 can then be formed by a single reflective metal layer, for example an indium-silver alloy. This makes it possible at the same time to obtain a good reflection coefficient of the photons, to take good quality electrical contact on the anode layer 103c, and to simplify the production of the structure 107 (with respect to the prediction of a mirror of Bragg).
Des modes de réalisation particuliers ont été décrits. Diverses variantes et modifications apparaîtront à l'homme de l'art. En particulier, on notera que les variantes de réalisation des figures 3 et 4 peuvent être adaptées au procédé des figures 2A, 2B, 2C.  Particular embodiments have been described. Various variations and modifications will be apparent to those skilled in the art. In particular, it will be noted that the variant embodiments of FIGS. 3 and 4 can be adapted to the method of FIGS. 2A, 2B, 2C.
De plus, les types de conductivité des couches de nitrure de gallium 103a, 203a3 (de type N dans les exemples décrits) et 103c (de type P dans les exemples décrits) , peuvent être inversés, les régions d'anode et de cathode des LEDs étant alors également inversées.  In addition, the conductivity types of the gallium nitride layers 103a, 203a3 (N type in the examples described) and 103c (P type in the examples described) can be reversed, the anode and cathode regions of the LEDs are then also reversed.
En outre, les modes de réalisation décrits ne se limitent pas aux exemples susmentionnés dans lesquels l'empilement actif de LED au GaN est réalisé sur un substrat de croissance 101, puis reporté sur un substrat de support 105. A titre de variante, le substrat de départ peut être un substrat de GaN autosupporté, par exemple dopé de type N, sur une face duquel est épitaxié l'empilement actif 103. A titre d'exemple, l'empilement actif 103 est formé sur la face de polarité azote du substrat. Dans ce cas des nanofils de GaN peuvent être formés directement sur la face de l'empilement actif opposée au substrat, qui est une face de polarité azote.  In addition, the described embodiments are not limited to the aforementioned examples in which the GaN LED active stack is made on a growth substrate 101, then transferred to a support substrate 105. Alternatively, the substrate starting point may be a self-supporting GaN substrate, for example doped N-type, on one side of which the active stack 103 is epitaxial. By way of example, the active stack 103 is formed on the nitrogen polarity of the substrate . In this case, GaN nanowires can be formed directly on the face of the active stack opposite to the substrate, which is a face of nitrogen polarity.
Par ailleurs, pour augmenter le couplage entre la zone d'émission et les nanofils et éviter que des photons ne sortent de l'empilement actif de LED entre les nanofils, un métal réfléchissant, par exemple de l'argent, peut être déposé sur la face supérieure de la couche 103a entre les nanofils 109, 409. A titre d'exemple, ce métal peut être déposé avant la croissance des nanofils sur toute la surface de la couche 103a, puis retiré localement dans les zones de croissance des nanofils. A titre de variante, le métal réfléchissant peut être déposé sur toute la surface de la LED après la réalisation des nanofils 109, 409, par exemple par une méthode de dépôt conforme, puis une étape de gravure directive peut être mise en oeuvre pour retirer le métal sur la face supérieure des nanofils 109, 409. Moreover, to increase the coupling between the emission zone and the nanowires and to prevent photons from coming out of the active stack of LEDs between the nanowires, a reflective metal, for example silver, can be deposited on the upper face of the layer 103a between the nanowires 109, 409. By way of example, this metal may be deposited before the growth of the nanowires over the entire surface of the layer 103a, and then removed locally in the growth zones of the nanowires. Alternatively, the reflective metal may be deposited on the entire surface of the LED after the production of the nanowires 109, 409, for example by a conformal deposition method, then a directional etching step may be implemented to remove the metal on the upper surface of nanowires 109, 409.
En outre, à titre de variante, dans le cas où les nanofils 109 sont en GaN, on peut, en modifiant les conditions de l'épitaxie, faire croître autour des nanofils 109 des coquilles contenant des puits quantiques adaptés à convertir dans une autre couleur une partie de la lumière émise par la couche 103b. A titre d'exemple, la couche 103b peut être adaptée à émettre de la lumière bleue, et des puits quantiques adaptés à convertir en lumière jaune une partie de la lumière bleue émise par la LED peuvent être formés autour des nanofils 109, de façon à obtenir une LED émettant de la lumière blanche.  In addition, as an alternative, in the case where the nanowires 109 are GaN, it is possible, by changing the conditions of the epitaxy, to grow around the nanowires 109 shells containing quantum wells adapted to convert into another color a part of the light emitted by the layer 103b. For example, the layer 103b may be adapted to emit blue light, and quantum wells adapted to convert yellow light part of the blue light emitted by the LED may be formed around the nanowires 109, so as to get an LED emitting white light.

Claims

REVENDICATIONS
1. Procédé de fabrication d'une diode électro¬ luminescente au nitrure de gallium, comprenant les étapes successives suivantes : 1. Process for manufacturing a gallium nitride electroluminescent diode, comprising the following successive steps:
a) former par épitaxie, sur un substrat de croissance (101) , un empilement actif planaire (103 ; 203) de diode électroluminescente au nitrure de gallium comportant, dans l'ordre en partant du substrat de croissance, une première couche de nitrure de gallium (103a ; 203a3) dopée de type N, une couche émissive à un ou plusieurs puits quantiques, et une deuxième couche de nitrure de gallium (103c) dopée de type P, la face de la première couche de nitrure de gallium (103, 203a) tournée vers le substrat de croissance étant de polarité azote ; a) forming by epitaxy, on a growth substrate (101), a planar active stack (103; 203) of gallium nitride light-emitting diode comprising, in order starting from the growth substrate, a first layer of gallium nitride gallium (103a; 203a3) doped with N type, an emissive layer with one or more quantum wells, and a second layer of gallium nitride (103c) doped with P type, the face of the first layer of gallium nitride (103, 203a) facing the growth substrate being of nitrogen polarity;
b) reporter l'empilement actif planaire (103 ; 203) sur un substrat de support (105) et retirer le substrat de croissance (101) de façon à libérer l'accès à ladite face de polarité azote de la première couche de nitrure de gallium (103a ; 203a3) ; et c) faire croître des nanofils de nitrure de gallium (109) par épitaxie en phase vapeur sur ladite face de polarité azote de la première couche de nitrure de gallium (103a ; 203a3) . b) transfer the planar active stack (103; 203) onto a support substrate (105) and remove the growth substrate (101) so as to free access to said nitrogen polarity face of the first nitride layer of gallium (103a; 203a3); and c) growing gallium nitride nanowires (109) by vapor phase epitaxy on said nitrogen polarity face of the first gallium nitride layer (103a; 203a3).
2. Procédé selon la revendication 1, comprenant en outre, entre l'étape a) et l'étape b) , une étape de formation d'une structure réfléchissante (107) sur la face de la deuxième couche (103c) de nitrure de gallium opposée à la couche émissive (103b) ou sur le substrat de support (105) , de façon que, après l'étape b) , la structure réfléchissante (107) fasse interface entre la deuxième couche (103c) de nitrure de gallium et le substrat de support (105), 2. Method according to claim 1, further comprising, between step a) and step b), a step of forming a reflective structure (107) on the face of the second layer (103c) of nitride gallium opposite the emissive layer (103b) or on the support substrate (105), so that, after step b), the reflective structure (107) interfaces between the second layer (103c) of gallium nitride and the support substrate (105),
dans lequel la structure réfléchissante (107) est un miroir de Bragg comportant uniquement des matériaux ayant un point de fusion supérieur à 1100°C. in which the reflecting structure (107) is a Bragg mirror comprising only materials having a melting point greater than 1100°C.
3. Procédé selon la revendication 1 ou 2, dans lequel l'épaisseur de la première couche (103a ; 203a3) de nitrure de gallium entre la couche émissive (103b) et la base des nanofils (109 ; 409) est inférieure à la longueur d'onde d'émission de la couche émissi e (103b) . 3. Method according to claim 1 or 2, in which the thickness of the first layer (103a; 203a3) of gallium nitride between the emissive layer (103b) and the base of the nanowires (109; 409) is less than the emission wavelength of the emitted layer (103b).
4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel les nanofils (109) ont un diamètre compris entre 50 et 250 nm, et une longueur comprise entre 0,5 et 15 um. 4. Method according to any one of claims 1 to 3, in which the nanowires (109) have a diameter of between 50 and 250 nm, and a length of between 0.5 and 15 µm.
5. Procédé selon l'une quelconque des revendications 1 à 4, dans lequel les nanofils (109) ont une forme tronconique de diamètre augmentant progressivement en s 'éloignant de la première couche (103a ; 203a3) de nitrure de gallium. 5. Method according to any one of claims 1 to 4, in which the nanowires (109) have a frustoconical shape of diameter gradually increasing away from the first layer (103a; 203a3) of gallium nitride.
6. Procédé selon la revendication 5, dans lequel, pour obtenir ladite forme tronconique, la température d'épitaxie est progressivement réduite au fur et à mesure de la croissance des nanofils (109) . 6. Method according to claim 5, in which, to obtain said frustoconical shape, the epitaxy temperature is gradually reduced as the nanowires (109) grow.
7. Procédé selon la revendication 6, dans lequel ladite température d'épitaxie est progressivement réduite depuis une température de l'ordre de 1050°C en début d'épitaxie jusqu'à une température de l'ordre de 900 °C en fin d'épitaxie. 7. Method according to claim 6, in which said epitaxy temperature is gradually reduced from a temperature of the order of 1050°C at the start of epitaxy to a temperature of the order of 900°C at the end of the epitaxy. epitaxy.
8. Diode électroluminescente au nitrure de gallium, réalisée par un procédé selon l'une quelconque des revendications 1 à 7. 8. Gallium nitride light-emitting diode, produced by a process according to any one of claims 1 to 7.
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